Phase modulation synthesis as Yamaha does it is computationally very cheap. That was one of the things that really boosted its fortunes in the 80s: you could produce very complex timbres (albeit often nasal or harsh sounding) with just a few sine waves pushing each other around. It's literally just a sine wave whose output pushes the phase of the next sine wave (operator in Yamaha's parlance) forward. You don't even need floating point math, and Yamaha got by with 12-bit DACs.

The SY99/77 really are great, though. I sold my TG77 and FS1R recently, but will probably get myself a used SY99 keyboard at some point. Or save up for a Montage.